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help for qweibull 
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Quantile-quantile plot for data versus fitted Weibull distribution

        qweibull varname [if exp] [in range] [weight] [, grid param(# #)
                         show(condition) generate(newvar) graph_options ]

    where graph_options are

        rlopts(cline_options) plot(plot) addplot(plot) scatter_options
        twoway_options

    fweights and aweights are allowed; see help weights.


Description

    qweibull plots the quantiles of varname against the quantiles of a
    two-parameter Weibull distribution with probability density function for
    variable x >= 0, scale parameter b > 0 and shape parameter c > 0 of (c/b)
    (x/b)^(c - 1) exp(-(x/b)^c).  By default, maximum likelihood estimation
    is carried out by weibullfit.


Remarks 

    In the majority of cases, qweibull will be used to fit an Weibull
    distribution on the fly and to assess that fit. In some cases, it may be
    of interest to compare data with a Weibull distribution with known or
    hypothesised parameters, which may be specified using the param() option.
    In all cases it is important not only to specify any if or in
    restrictions, but also to specify relevant weights, which will (unless
    constant) affect the configuration of the plot.


Options

    grid adds grid lines at the .05, .10, .25, .50, .75, .90, and .95
        quantiles.

    param() may be used to supply parameter values (namely, scale and shape
        parameters) directly for use in estimation of fitted quantiles.  b
        and c should be provided as separate values in precisely that order.

    show() may be used to specify that you wish to restrict the graph
        according to some condition, say looking at one tail of the
        distribution only. Note that if and in should not be used for this
        purpose.

    generate() specifies the name of a new variable to hold values of fitted
        Weibull quantiles.

    graph_options are rlopts(cline_options), plot(plot), addplot(plot),
        scatter_options, and twoway_options.

    rlopts(cline_options) affect the rendition of the reference line; see
        help cline_options.

    plot(plot) provides a way to add other plots to the generated graph; see
        help plot_option (Stata 8 only).

    addplot(plot) provides a way to add other plots to the generated graph;
        see help addplot_option (Stata 9 up).

    scatter_options affect the rendition of the plotted points; see help 
        scatter.

    twoway_options are any of the options documented in help twoway_options
        excluding by().  These include options for titling the graph (see
        help title_options) and options for saving the graph to disk (see
        help saving_option).


Examples

    . qweibull spacing

    . qweibull spacing, param(20 3)

    . qweibull spacing, gen(spacing_w)


Author

    Nicholas J. Cox, Durham University, U.K.
    n.j.cox@durham.ac.uk


References

    Evans, M., Hastings, N. and Peacock, B. 2000. Statistical distributions.
    New York: John Wiley.

    Johnson, N.L., Kotz, S. and Balakrishnan, N. 1994.  Continuous univariate
    distributions: Volume 1. New York: John Wiley.

    Kleiber, C. and Kotz, S. 2003.  Statistical size distributions in
    economics and actuarial sciences.  Hoboken, NJ: John Wiley.


Also see

    Online:  help for pweibull (if installed), weibullfit, graph